1. Field of the Invention
The present invention relates to a color image forming apparatus, such as a color copier, a color printer, a color facsimile apparatus, a color printing apparatus, etc., forming a full color image by superimposing images of individual colors on top of one another, and in particular relates to control of aligning positions of images of individual colors that are superimposed on top of one another to form a full color image.
2. Description of the Background Art
In a color image forming apparatus forming a full color image, images of cyan, magenta, yellow, and black are superimposed on top of one another and thereby a full color image is formed. If positions of images of individual colors (cyan, magenta, yellow, and black) superimposed on top of one another to form a full color image are deviated from each other, colors of a line and a character of the full color image change or irregular colors occur in the full color image, leading to deterioration in image quality. Accordingly, in forming a full color image, it is necessary to align positions of images of individual colors that are superimposed on top of one another to form the full color image with each other as much as possible.
Japanese Patent Laid-open publication No. 63-286864 describes an image forming apparatus forming a full color image by superimposing images of individual colors on top of one another, in which deviations in position of images of individual colors (cyan, magenta, yellow, and black) in a main scanning direction (a direction perpendicular to a direction in which a recording sheet and a transfer belt are conveyed) are corrected. Such deviations in position of images of individual colors occur due to various reasons such as changes in external and internal environmental temperatures and changes in the apparatus over time. In the image forming apparatus, for correcting deviations in position of images of individual colors in the main scanning direction, a full color pattern image of a pattern of straight lines (extending in the main scanning direction) and slanted lines (extending slantingly relative to a transfer belt conveying direction) is formed on the transfer belt by superimposing pattern images of individual colors on top of one another. The straight lines and the slanted lines of the pattern of lines of the pattern image formed on the transfer belt are detected by sensors, values of distances between the straight lines and the slanted lines of the pattern of lines of the pattern image formed on the transfer belt are obtained according to signals from the sensors, and based on the values of distances between the straight lines and the slanted lines of the pattern of lines of the pattern image formed on the transfer belt and reference values stored in a memory, amounts of deviations in the positions of the pattern images of individual colors in the main scanning direction are calculated by a CPU. Based upon a result of such calculation by the CPU, at least start timings of writing images on photoconductors in the main scanning direction or frequencies of writing clocks are corrected. Thus, deviations in position of images of individual colors in the main scanning direction are corrected, and thereby a full color image with no deviation in position of images of individual colors is obtained.
In the above-described color image forming apparatus, a pattern image of a pattern of straight lines and slanted lines of individual colors (cyan, magenta, yellow, and black) is formed on a transfer belt, the straight lines and the slanted lines of the pattern of lines of the pattern image formed on the transfer belt are detected by sensors, and amounts of deviations in position of the pattern images of individual colors are obtained based upon detection signals of the straight lines and the slanted lines of the pattern of lines of the pattern image formed on the transfer belt from the sensors to be fed back to a correction part of the apparatus so that the deviations in position of the pattern images of individual colors are corrected.
Accordingly, a density of the pattern image formed on the transfer belt must be on a sufficient level that each of the straight lines and the slanted lines of the pattern of lines of the pattern image can be detected with the sensors. If the pattern image formed on the transfer belt is blurred or includes a thin spot, that is, if any of the straight lines and the slanted lines of the pattern of lines of the pattern image is blurred or includes a thin spot, it may occur that the amounts of deviations in position of the pattern images of individual colors obtained based upon the detection signals from the sensors are not correct. In this case, naturally, the deviations in position of pattern images of individual colors cannot be correctly corrected, leading to deterioration in image quality. Further, by performing the operation of correcting deviations in position of images of individual colors uselessly, the printing speed of the apparatus is decreased. By performing without condition a process control (control for optimizing image forming conditions of the apparatus by detecting image density, etc.) before executing the operation of correcting deviations in position of images of individual colors, the density of the pattern image formed on the transfer member can be increased so that amounts of deviations in position of pattern images of individual colors can be correctly obtained, and thereby the deviations in position of pattern images of individual colors can be correctly corrected. However, it is more preferable to minimize the time other than the time of printing images to suppress a lowering of the printing speed of the apparatus.